The existing frame has the gambrel roof with plywood gussets, the house is to have a straight, pitched roof(9/12).
Once I see an estimate on the trusses, I'll be in a better position to figure out the cheaper route? Believe me, i would love to cut a queen/purlin system for the frame but cost is the key here.
I agree that a 5' kneewall would provide enough headroom, my concern is that the 5' kneewall would be difficult to engineer, even with queens and purlins in place.
Conventional trusses should eliminate roof thrust from the frame and then we only have to worry about the top of the sidewalls having enough stiffness to carry the trusses...which could be a problem in itself as the bay plan will be 18'/4'/18'.
However the walls are intended to be strawbale so it will be possible to place vertical posts against the outside of the existing frame(buried in the strawbale), rather than cut through the girts for intermediate sidewall posts.